1/4 Scale servos ?

[KenC]

Of all the brands and styles, KG ratings and voltages, what do our experts recognise as the "bangs for buck "  choice in the larger scale arena.   Are Hitec HS 485HB adequate, and which functions might require a tad more ?   Whats the maximum practical distance a servo can be sited from an elevator before issues arise ?

[Jon H]

It all depends on the specific model you are flying as large scale can cover anything from a wright flyer to an F35. Needless to say they behave differently! 

 

 

Which model are you looking to equip? might be able to give more helpful info

[KenC]

Hi Jon, thanks for the message, I am building a Top Flite Cessna 182 from a Gold Kit  with 81" span . Presumed all up is around 10 to 12 lbs.

[Martin Harris - Moderator]

I would have thought that assuming you're powering it sympathetically, any good quality standard servo would be adequate for this type of model. 

 

Linkage length would depend on strength and stiffness of the materials used.

[KenC]

Do you feel a metal geared servo might be more applicable for the  shared nose wheel and rudder, than the Hitec I mentioned  Martin?   The length of the wire rods in sleeves fed through the side members of the fuse is around 40".

 

[Martin Harris - Moderator]

That’s a good call for the nose wheel servo depending on whether there’s any form of servo saver in the linkage. 
 

The long wire linkages’ efficiency will depend on how straight the runs are and how well the ends of the supporting tubing are fixed plus how good the running clearances in the outer tubes are. 
 

I prefer something like carbon tubes for pushrods - light and stiff - with proper through bolted ball links but they may be difficult to retrofit into an ARTF. 
 

Pull-pull cables are also a good choice where practical. 

[stu knowles]

HiTec 645 servos have stood me well in many large models.  Robust and good value.  As with the radio, the choice is huge

[Nigel R]

I like hitec. I would pick the 645 for anything one step up from a regular servo. Your cessna feels like it needs something just a bit more than a standard.

 

Or another option. If you already have some standard servos available. You could use one standard servo per elevator, one per aileron, one per flap. Rudder would be easiest with one bigger unit (although it is possible to gang servos together with the right linkages).

 

 

[Jon H]

The TF 182 is a really nice model and standard servos would be fine throughout with no concerns. 

 

The hitec 645 is a fine servo, i have some, but its wild overkill for this model. If you wanted a bit of an upgrade the savox 351 gets my vote as its a little more torque than standard, has digital holding power and is generally a more cost effective solution as you can get two for the price of one 645. The hitec 485 isnt bad either although there is debate about how good the karbonite gears are vs nylon or metal. If this were an aerobatic model or WWII fighter of the same size with a 30-50cc type powerplant then the 645 would be fine, still over spec to some degree but fine. I am just not seeing a need for that amount of power on a cessna this size. 

 

When it comes to the nosewheel steering you can fit servo saver spring things but i have to admit i have never found it necessary. In my experience the most common cause for damage to nosewheels and their linkages is people pushing the model into the runway nosewheel first when landing, rather than holding the model off as long as possible and letting it settle into the main wheels first. You can also protect it by doing all the things full size pilots do like taxi with up elevator held in to unload the nosewheel. 

 

I would also not be worried about long wire pushrods. IF they are of the right wire and are supported nicely along their length there will be no problem at all. 

 

My final advice would be not to over power the model. It might look big but i would strongly recommend you resist the urge to fit a massive engine as its supposed to fly with its wings and not its propeller. I have customers who have used our 100 and 120 over the years, one used a 160v with a large 3 blade prop to tame the beast (it was over powered) but any decent 90-120 4 stroke will work just fine. Not sure what the equivalent electric setup would be but again, no need to go mad. 

[KenC]

Thanks all for your input,  I was amazed that the kit designer did not offer indication of the type of servo to be used..... after all the throws etc are charted . 

 It is my plan to have 4 servos in the wings  and "Y" split for two channels , but Jon's comment over carbonite gears does make me wonder about a larger Savox  metal gear servo for the shared rudder and steering wheel.  As for power, this is looking like being my first foray into electric with a spec by 4Max  to include :-

 

16x8 on 6S 5000mAh LiPo on the PO-5065-360 then

if flown in a scale like manor I would expect flight times around

8-10 minutes depending upon weather conditions and throttle usage.

This setup should provide around 12lbs of static thrust with an

estimated top speed of around 58MPH

[steve too]

12 hours ago, KenC said:

I am building a Top Flite Cessna 182 from a Gold Kit  with 81" span.

 

If I were buying servos for that model, I would probably go for Savox SC-0253MG all around with horns and ball links from QuickUK.

 

If I was fitting it out from my spares box, it would be getting Hitecs (485s and 645s), but Savox are better priced at the moment.

[KenC]

Yes the £30 a pop for the Hitec  645 is a tad high  compared to £20 for the Savox .    Digital servos are relatively new to me , all we used in the old days were the Futaba 300 series .   Is there any issue mixing digital with analog  servos , other than the obvious reaction time settings that would need to be kept lower.

[Jon H]

50 minutes ago, KenC said:

Jon's comment over carbonite gears does make me wonder about a larger Savox  metal gear servo for the shared rudder and steering wheel.

 

yea there has been discussion about it for years with many reporting that even the nylon gears are better at shock resistsnce. Remember too that the more you beef up one thing, the more likely to are to break something else. I have never stripped servo gears on a nosewheel so i just dont worry about it...in fact i have never stripped servo gears in normal operation. Only crashes and clumsiness in the hangar have caused issues 

 

52 minutes ago, KenC said:

estimated top speed of around 58MPH

 

Too fast!!! At the revs i fly my models at (not full power) i get about 55mph for warbirds and 20-25 ish for my piper Tomahawk and Stampe. These are calculated figures from the pitch speed not measured figures. I did measure one of my warbirds once flat out and it was somewhere around 60-70mph. Didnt have a radar gun, just watched the shadow and did a distance/time calculation. 

 

In any case, the stall speed of the full size is only 45mph so near 60 for the model is much too fast

 

I would recommend no more than 6'' pitch on a model like this and, from very rough calculations using the recommended 90-120 4 stroke, 8-10lbs thrust is the intended level of power from topflite. 

[KenC]

Jon I would have no intention of belting it round the sky at 58 mph !  However I do like to operate with a margin of power in reserve, and like the idea of working at 50% of the heat possible in an enclosed cowl.    The choice of electric is a mixture of carrying all the starting kit and fuel  on the long hike at Headcorn Aerodrome  and the cleanliness of placing a non oily model over the leather seats of my car !   As for the power ratio, I read that the suggested OS 120 Surpass  is around the same 2 HP as my electric choice. 

[Martin Harris - Moderator]

1 hour ago, KenC said:

Thanks all for your input,  I was amazed that the kit designer did not offer indication of the type of servo to be used..... after all the throws etc are charted . 

According to the Horizon Hobbies website it requires “Servos: 6 standard with at least 44oz/in (3.2kg/cm) torque”

 

Hopefully their technical department is more clued up than their copywriters - it’s surprising how many people quote torque incorrectly - it should read 3.2 kg.cm 

[Nigel R]

If this fits with a 90 four stroke then I would use standard size servos.

 

Personally I've never had a problem with the 485 gears. I would generally prefer nylon gears over metal anyway.

[Frank Skilbeck]

Fitting separate servos for the rudder and nose leg steering on separate channels would let you trim the rudder and nose leg independently. I.e if you find you need some rudder trim in flight you can dial that in electronically and the nose leg trim isn't affected.

[KenC]

Martin  the kit is the older wooden build it kit rather than the newer ARTF foam one.  I think the sizes are similar though  with weights not too far apart. The book that comes with the kit is quite detailed with " tip of the day "  entries as you build......were the kit still in production I might have mailed my tip of the kit suggestion !

Frank that is an idea that has crossed my mind separating the servos and halving the workload.  Might just make it with   Ailerons, flaps, rudder, nose wheel, elevator, ESC,  lights,  AS3X/Safe    Be rather nice to control the takeoff with separate trim for wheel and rudder.

Edited May 1, 2023 by KenC

[Jon H]

5 hours ago, KenC said:

I read that the suggested OS 120 Surpass  is around the same 2 HP as my electric choice. 

 

A 120 propped to suit this model will spit out about 1.7hp not that it means much to measure it that way. Engine HP ratings a bit meaningless due to the way they are measured. 

 

In any event, having some power to spare is fair enough, but you have a good bit more than you need. 

[KenC]

Thanks for your input Jon , if you think this overpowering is likely to jeopardise  any part of the spec if used prudently please suggest areas to watch .

[Jon H]

2 hours ago, KenC said:

Thanks for your input Jon , if you think this overpowering is likely to jeopardise  any part of the spec if used prudently please suggest areas to watch .

 

no its more that its going to be more difficult to keep the speed down with a larger excess of power. I have flown customers Flair Pup's with our 70 fitted and they are horrible compared to the one my Dad has with a 50 4 stroke. I know people say you can always throttle back but there comes a point where you just cant any more and the model becomes unpleasant. 

 

That said, electric is a little different as you can turn it off completely where as an engine will always be running. 

[KenC]

Yes I get what you are saying , perhaps if we could takeoff from a bowling green  or formal paved runway where it allowed the security of a nice build up of speed pre rotation , less power would be more easily accepted ...... I adore watching talented modelers and fliers producing those scale takeoffs  where the plane rolls on two wheels pre climb with fuse horizontal ..... but on many club strips the desire is to become airborne before the jungle devours you !

[Paul De Tourtoulon]

Just looking up the HP rating of the OS 120, 2.1hp, the four strokes do vibrate a lot, but I don't see the point of metal gear servos for most planes, being

satisfied  Futaba user I would go S 3001 or if you need a bit more power for the rudder and elevator the s 3010 are worth the investment.

 

http://sceptreflight.com/Model Engine Tests/OS FS-120 Surpass.html

[Jon H]

2 hours ago, Paul De Tourtoulon said:

Just looking up the HP rating of the OS 120, 2.1hp

 

If you calculate it using the online calcs available then you get a peak of 2.4hp when using the 14x8 zinger, and 2.5 on the 15x6. but its doing 10700rpm for crying out loud. You never run a 120 that fast if you have any interest in noise and/or efficiency. Taking the slightly more sensible 15x8 at 8700rpm you get... 1.8, this drops to 1.7 on the 16x6 at 8600 and rises to 2 again on the 16x8 at 8400. Clearly the 16x8 is an outlier caused by an error in calculation somewhere. The revs are dropping off, so will power. 

 

This is a very quick graph plotting the RPM vs calculated power from the review using their rpm figures. The circle is where the 16x8 calculated value should sit, which is clearly wrong when viewing the overall trend. 

 

 

Quoted HP figures are meaningless as they are always quoted at much much higher rpm than you would ever actually use in practice. We also do not have the required constants in place when it comes to testing as props vary. Although the review states the fuel, manufacturers do not so they could have tipped some real hot juice in to get the power figures up. Unless the engine is dyno tested with proper procedures, its just a meaningless number and its why i dont give them out for Laser engines. I could, but they dont mean anything. Also as i would calculate them at normal operating RPM rather than revving it mad with a fruity fuel, they would look rubbish. I could make them look good, but that is a little dishonest is it not? 

 

Also, HP wont make the model move. Its all about the prop and its efficiency. Stick a tooth pick on there and rev the rod off the engine, you will see fantastic calculated power, but no thrust. Its thrust that we perceive as power so the prop is more important than anything else. The Zinger props in the review were fast and not brilliantly efficient, but offer great calculated HP. I am not suggesting it was anything devious on the part of the reviewer, just that the prop choice makes a massive difference to the performance. 

 

If you need further proof look back to the start of WWII. The Spitfire and Hurricane were both crippled by the early fixed pitch 2 blade prop and both performed more or less the same using the same engine. Better props improved their performance but allowed the more advanced Spitfire to out perform the Hurricane as it was no longer restricted by its propeller and the performance limiter became the airframe itself. The P47 was similar later in the war. 

 

 

10 hours ago, KenC said:

Yes I get what you are saying , perhaps if we could takeoff from a bowling green  or formal paved runway where it allowed the security of a nice build up of speed pre rotation , less power would be more easily accepted ...... I adore watching talented modelers and fliers producing those scale takeoffs  where the plane rolls on two wheels pre climb with fuse horizontal ..... but on many club strips the desire is to become airborne before the jungle devours you !

 

Using the more limited power i have been talking about i can still take off at half throttle with most of my models on a 'club field' if there is such an animal. My Tomahawk is an exception as its got a really draggy undercarriage and the high tail puts the elevator out of the propwash so i am unable to unload the nosewheel on the takeoff roll until the speed is quite high. Its also the only model i fly where i use a small amount of flap on takeoff. Generally i dont recommend it, but this model has some rather unique traits. 

 

I do appreciate your point though as you dont want to be eaten by a tiger as you rescue your model from the weeds. I would go for a large diameter prop with fine pitch. This will give high thrust, quick acceleration but a low maximum speed so ticks all the boxes. You might even be able to use something like a 15 or 16x6 3 blade. 

 

[john stones 1 - Moderator]

They paint floats pretty colours to attract anglers to part with their money, makes no difference as only the tip shows in use, numbers are pretty much the same in the modeling game, how's the model fly is what's relevant.